By JP Yousha, Danika Bannasch DVM,PhD, & Phyllis McDonald
INTRODUCTION: This article has been written to offer breeders of the Nova Scotia Duck Tolling Retriever a "hands-on" approach to coat color genetics and to acquaint readers with the more recent advances in coat color research, especially as they apply to the Toller. We are not "there" yet: we don't have all the answers today, but we are now on the road to answering completely and accurately all these old and persistent questions of how coat color is inherited. Breeders should keep abreast of current research so that they can breed knowledgeably. This article will start with needed terminology and then quickly move into a discussion of the various color genes, outlining known facts and recent advances. Some notes on the tradition and history of color in the Toller and a few comments on the relevance of color to a breeding program are offered in conclusion.
Let's begin with a bit of basic terminology. We must first define a few terms just to put them in place and get them out of the way. A LOCUS is simply the location of a gene; the place it lives on a particular chromosome-its street address if you will. LOCI is the plural. An ALLELE is one particular form of a gene at any given locus. Essentially you have a wild type allele (the "standard" gene), and any variation is then defined as a mutation. A mutation can be an improvement in some way, a simple variation, or can sometimes be detrimental, but is not necessarily so. Some mutations are "dominant" and many mutations (and more typically in purebred dogs) are "recessive." A dominant allele will change the dog's color when the dog has just one copy while a recessive allele requires two copies to change the dog's color. Some alleles are somewhere in between and are called "incomplete dominants" or "co-dominants." This means that the trait is seen with just one copy but that it is more pronounced if you have two copies. Some genes have a specific effect all on their own, but many are altered by the affects of genes at other locations. A phenotype, (or what you see) is the result of their combined action (and various non-heritable conditions as well).
Mutations for pigment produce the variations in coat color that we see, as well as some defects in dogs. Genes named under the old "Little" system typically were identified by what sort of effect the mutation had. The M Locus, for example, was named for the "merle" gene, as here was a dominant mutation that caused a solid coat to appear patched or dappled. The C Locus was named for the "chinchilla" gene, which is a recessive mutation that washes out a rich gold coat to a pale cream color. Under the new system, now emerging, genes are being named in an even more cryptic manner, but the newer names are more accurate and reflect new molecular data. So we shall all have to get used to them. The old Extension or "E" locus, for example, is now referred to as MC1R. This sounds rather fearsome to most of us already a bit bemused by the "E locus," but is simply "gene speak" for the actual gene (the melanocortin receptor 1) that has been found to cause the effects we associate with the E Locus. So this is a step forward really, and when you see these sorts of names that means the actual gene has been found and the action of the gene is known. Knowing this might make it easier to try to remember the new names. This is real progress, as it represents the first time we have real knowledge of the genes involved, and each discovery is quickly producing a DNA test to identify the dogs that carry that mutation. That translates into breeder power, as you will then be able to select the dog you want at the direct, genetic level.
One more quick note on nomenclature: pigment in dogs comes in two basic forms. Eumelanin is the dark pigment that is black, brown (i.e. chocolate) and blue by our breeder terminology. The other pigment is phaeomelanin and this is the bright pigment that produces red, yellow, and cream colors in canine coats. Each type of pigment is affected by different genes. Some genes function to affect the intensity of the pigment and so change jet black eumelanin of the hair coat to the warm brown tone called "chocolate" in breeds like the Labrador Retriever, and causes the nose on Tollers to change from black to a brown that is referred to as "self." Other genes are "pattern" genes and alter the distribution (not the color) of the pigment; like sable, which gives you a darker overlay (the eumelanin) over a brighter coat (the phaeomelanin). When you have more than one mutation, you can get more than one effect on the same dog. For example, you can have a normal sable Toller with either a black overlay and a black nose or a brown sable Toller with a brown or "self" nose. In the latter case both the brown pigment mutation and the sable pattern mutation are acting to produce the final effect. Note also that "white" genes are actually genes which disable the body's ability to make pigment, as areas of white actually LACK pigment (i.e. white is not a "third" form of melanin). All white genes in the Toller are actually spotting genes.Melanocytes are the pigment producing cells of the body. The coat color genes can be divided into two groups; those that control the development, differentiation and distribution of melanocytes and those that directly affect pigment synthesis. The mature melanocytes are located at the base of the hair follicles where they synthesize pigment and release it into the hair shaft. Absence of the melanocytes leads to white spotting. As you can imagine there are lots of different genes that can mess up the melanocytes being in the right place There is a separate set of genes whose protein products are important for proper deposition of pigment into the hair follicle. If the pigment is not evenly distributed along the hair shaft then it changes the color of the dog's coat. Dilute coat color is caused by pigment clumping in the hair shaft.
When thinking of a gene, try to not think of the dog you see so much (i.e. try to forget the phenotype for a moment), and start to think about the biology of the thing-what the gene does to make the color or pattern we see. For each gene there is a specific molecular action taking place. Thinking this way (rather than thinking "there is no buff in my pedigree!") will help you understand not only the new information on coat color, but help you learn about genetics and how disease and inherited traits in general actually occur. Each functioning gene is preserved DNA code that results in the formation of a protein. Proteins literally build bodies. Mutations alter the protein formed, so the changes you see are a result of the gene acting differently due to the alterations that occurred to it on a (sub)-molecular level. Isolating this change is the first step to identifying and controlling the gene in question. Traits that are "genetic" (that is heritable) are ultimately under our control given the right tools, so defining a trait as heritable is less dooming an individual to its fate than offering us all a hopeful message that we are going to be able to select just what we want in our dogs. That's another change we need to make in our mentality about genes: knowing a trait is inherited is GOOD news, even if the trait is a disease or unwanted color! So be glad a trait is inherited, as that means we can ultimately choose to have it or not have it in our dogs.Variations in hair color can arise from either differences in pigment synthesis, or in the deposition of pigment. Hair shafts can contain black, brown, gray, red, yellow, or colorless pigment granules, where the color of the granule is dependent on the kind and amount of pigment inside it. Each hair shaft can contain greater or lesser amounts of granules, thus producing variation in shade and intensity of color. The color and amount of granules can vary between hairs and along the length of the hair shaft in a single hair.
Now on to the specific genes associated with coat color: but please one quick note first. We wish to acknowledge that we have simply ignored various canine loci postulated (e.g. G Locus) that are not relevant to the Toller. The complete list of known canine coat color gene loci is so widely published it's easily found if needed. However, be warned that much of the tradition repeated so widely on coat color in dogs is so outdated and/or so inaccurate, any such source (especially internet layman sources) should be treated with some caution.
OUTLINE OF RED COAT COLOR GENES (as applies to the Toller):
Illustration of how sable fades from infancy to the adult dog.
Sable pup at around one week and the same pup at three months.
This is the same pup at 12 months.